According to one previous proposal, a valve member which is urged by a helical spring into a seating disposition with respect to a valve port is displaced from said seating disposition, to open the valve, upon translational movement of a diaphragm connected to the valve member by a valve stem or pivotable link which extends perpendicular to the valve seating surface. See U.S. Pat. No. 3,481,333 to Garrison. While such arrangements can provide a useful lever ratio in which a small pressure difference across the diaphragm can provide a large force to unseat the valve member, nevertheless the presence within the valve of moving parts and mechanical connections introduces source of unreliability, which are of course extremely undesirable when the correct functioning of the valve is essential to sustain life.
It has also been proposed to use as demand valves in breathing apparatus relatively simple diaphragm valves in which translational movement of the diaphragm in response to pressure differences across it causes a closure surface on the diaphragm to move between a disposition in which it is seated on a valve port and an open disposition in which it is spaced from the valve port. See U.S. Pat. No. 4,161,947 to Copson. Although the absence in such valves of a link or valve stem as mentioned above may endow the valves with greater reliability than the demand valves first above described, these latter valves lack a lever ratio for providing the desired sensitity of operation.